[patch,testsuite,avr]: Support dg-require-effective-target avr_3byte_pc (and use it with PR71151 tests).

[Mike Stump]

On Jun 22, 2016, at 7:21 AM, Georg-Johann Lay <avr@gjlay.de> wrote:
> 
> Some tests for PR71151 assume that the target MCU has a 3-byte PC.  The tests are failing because the simulator (avrtest) rejects to load the respective executables if .text exceeds 128KiB, e.g. for -mmcu=atmega128 which has only flash of 128KiB and only a 2-byte PC.
> 
> Hence the tests have to be skipped if the target MCU has no 3-byte PC, hence a new dg-require-effective-target proc supporting "avr_3byte_pc".
> 
> I added the new proc right after the last check_effective_target_arm_*** so that the test is in ASCII collating order.
> 
> Ok for trunk and v6?

No.  Please see target-utils.exp and ensure that the tools generate a stylized message and then add support for that to target-utils.exp.  If you are using binutils, the text should go into a memory segment that will fill when it is too large.  When it does, then binutils will generate one of the messages already handled, then you're done.